Miocene to recent alkaline volcanism between Al Haruj and Waw an Namous (southern Libya)

Unspiked K-Ar ages, petrological, geochemical and isotopic data are reported on samples from southern Libya (Wan an Namous-Al Haruj area). The Wan an Namous intracaldera cone dated at 0.2 Ma consists of unusually undersaturated foidite, representing the most extreme compositions among Libyan and Tibestian lavas. A basanitic and a basaltic lava flow located north-west of Wan an Namous, and probably belonging to the Al Haruj volcanic field, were dated at 5.1 Ma and 8.1 Ma. These data extend the range of ages previously reported for Al Haruj lavas. REE and multi-element patterns are typical of alkaline intraplate magmas. Sr ratios range from 0.70314 to 0.70812, whereas Nd ratios are very homogeneous (0.51290-0.51293). Pb ratios (19.231 < Pb-206/Pb-204 < 19.547, 15.607 < Pb-207/Pb-204 < 15.641 and 38.859 < Pb-208/Pb-204 < 39.242) are typical of HIMU-FOZO compositions. Such isotope characteristics are very similar to those available on two Gharyan (northern Libya) lavas and largely overlap those of Hoggar and Cameroon Line alkaline rocks. These lavas were produced by low and variable degrees of partial melting of a garnet- and amphibole-bearing mantle source, constraining the depth of melting between 80 and 150 km. Crustal contamination was also probably involved for the oldest sample. Mineral compositions of a dunite-harzburgite xenolith clearly indicate that the lithospheric mantle was affected by partial melting and metasomatic processes by magmatic liquids, probably associated with the genesis of Cenozoic lavas. Lithospheric delamination and asthenospheric upwelling, due to the reactivation of lithospheric megastructures induced by the Africa-Europe convergence, could represent a model for the genesis of Libyan lavas, as in Hoggar.